import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

import numpy as np

NA = np.fromstring(read(), np.int64, sep=' ')

A = NA[1:]
A[A < 0] = 0
g = np.gcd.reduce(A)

if g == 0:
    print(-1)
else:
    print(g * g)